@bronzegoddess "from zero" is not a solution for all possible sine curves. For example, sin(x)+1, "from zero" has an amplitude of 2, whereas in reality it's 1.
@jim_thompson5910 yeah that's right but not rigidly defined.

@vf321 its a simple way to think about it sorry if I am wrong.. if I was given an example like sin(x)+1 I would know that the amplitude is 1 because Asinx(Bx-C)+D, the amplitude is the number before the sin.